Salt water pools have MUCH lower salinity than our tanks. The seals on spa pumps won't hold up to our higher salinity salt water and will end up leaking. Also, the shafts in a spa pump likely won't hold up to the corrosive nature of high salinity salt water. While they would have stainless shafts, there are different grades of stainless and a cheap spa pump would have a salt water safe grade. There are however pond pumps like the Lim pumps that have salt water seals but those are modified specifically for aquarium use. I used Lim Wave pond pumps for many years but those had salt water seals. A typical spa pump puts out thousands of GPH. You'd need a pretty big mixing tank (300 gallons or so) to make that practical as a spa pump would create a serious torrent in a smaller tank like a 50 gallon brute or something like that. I have a 100 gallon mixing tank and even with that size tank, a spa pump would be way overkill. Spa pumps also use 1.5" - 2" fittiings which can be problematic with round tanks as large bulkheads like that would have a hard time sealing to the round tank. With a square or rectangular tank with flat surfaces, that wouldn't be an issue. While you can reduce the plumbing size on the outlet side, you don't want to reduce it on the inlet side as that can cause premature failure of the pump motor. Bottom line... While they may be cheap, they aren't a great choice for a mixing tank though they could work, they won't last and would result in much more engery consumption than needed. Then again, they only need to run a couple hours a day if that but the seals will fail before long unless you replace the seals with salt water safe seals.
